Aghdam, A.M., F. Karimi, and A. Najafizadeh. 2013. “ON THE SUBGROUPS OF TORSION-FREE GROUPS WHICH ARE SUBRINGS IN EVERY RING”. Italian Journal of Pure and Applied Mathematics 31 (December):63-76. https://journals.uniurb.it/index.php/ijpam/article/view/6160.